Electrical storage and delay circuits



Sept. 1, i942. B. M. HADFIELD 2,294,863

ELECTRICAL STORAGE AND DELAY CIRCUITS Filed March 29, 1941 2Sheets-Sheet l B TC2 P CY MR5 5 MR2 4 Q4 B Tea 8 l Sept. 1, 1942. a. M.HADFIELD ELECTRICAL STORAGE AND DELAY CIRCUITS Filed March 29, 1941 2Sheets-Sheet 2 IZbf' Patented Sept. 1, 1942 ELECTRICAL STORAGE AND DELAYCIRCUITS Bertram Morton Hadfield, Harrow Weald, England, assignor toAssociated Electric Laboratories, Inc., Chicago, Ill., a corporation ofDelaware Application March 29, 1941, Serial No. 385,956 In Great BritainApril 6, 1940 4 Claims.

The invention relates to electrical delay circuits and is moreparticularly concerned with circuit arrangements for time-delaying bystorage a sequence of voltage/time functions.

The series of voltage/time functions may be for instance, a number ofpulses of uni-directional voltage, the waveform of which may have aconstant or variable amplitude with time but the pulse is distinguishedby the time interval between points of zer voltage on the waveform. Thevoltages which'are delayed by storage in the circuit may be a. functionof the pulse amplitude or of the pulse time or both, but their sequenceis similar to that of the original pulses. It will be appreciated thatby delaying the sequence of any number of pulses it is possible tocompare with the voltage due to a pulse with the voltage due to anyother pulse whether the latter occurred before or after the former inthe original sequence.

According to one feature of the invention the circuit arrangementscomprise one or more branch circuits each including a uni-directionaldevice, a condenser in series therewith, the con denser being charged bymeans of the rectifier in dependence upon the voltage/time function andmeans controlled by the voltage/time function for discharging thecondenser to pass on the charge to subsequent equipment, the arrangementbeing such that the discharging of the condenser is completed at orbefore the time of arrival of the next voltage/time function at thatcondenser.

The unidirectional device may be for instance a dry plate rectifier orthe grid/cathode circuit of a thermionic valve and a single branchcircuit may be employed in which case the voltage developed by thedischarge of the condenser is effective on the controlled apparatus forinstance an indicating instrument of suitable type or a plurality ofbranch circuits may be employed in which case the voltage developed bythe discharge of the condenser is effective on the condenser of the nextbranch circuit while the voltage developed by the discharge of thecondenser of the last branch circuit is effective on the controlledapparatus. Further where a plurality of branch circuits are employed,the discharging of one condenser to charge the condenser of the nextbranch circuit prior to or at the time of arrival in United StatesPatent No. 2,207,513, issued July 9, 1940. This apparatus is arranged togive a visual indication of the percentage make and break p'eriod and/orimpulsing speed of an impulsing contact by simultaneously applyingvoltages substantially proportional to the logarithm of the make andbreak periods of the contact to the co-ordinate deflection plates of acathode ray tube. For this purpose it is necessary to delay theapplication of the voltages until both the make and break periods of oneimpulse have elapsed.

According to this feature of the invention, circuit arrangements adaptedto respond to a series of direct current impulses-and to renderavailable simultaneously voltages proportional to the make and breakperiod of each impulse successively comprise a first group of branchcircuits responsive to voltages proportional to the make periods of theimpulses and a second group of branch circuits responsive to voltagesproportional to the break periods of the impulses,'each branch circuitincluding a rectifier, a condenser in series therewith and means fordischarging the condenser after an appropriate time interval, the'number of branch circuits in the two groups differing by one and themeans for discharging the condensers of the first group being effectivesubstantially at the termination of a subsequent make period of theparticular impulse in order to pass on the voltage to the next branchcircuit while the means for discharging the condensers of the secondgroup is effective substantially at the termination of a subsequentbreak period in order to pass on the voltage to the next branch circuit.

of the next voltage/time function at the first The invention will bebetter understood from the following description taken in conjunctionwith the accompanying drawings inwhlch Fig. 1 shows the invention in itssimplest form,

Fig. 2 shows the invention applied to the delay of voltages representingthe make periods of a series of impulses,

Fig. 3 which should be taken in conjunction with Fig; 2 shows theinventionapplied to the delay of voltages representing the break periodsof a series of impulses,

Fig. 4 shows the use of thermionic valves for discharging thecondensers, I

Fig. 5 shows an arrangement for delaying the discharge of a condenserthrough a valve,

Fig. 6 shows an arrangement suitablev for use with an indicatinginstrument having a time lag comparable with the time spacing of thevoltage/ time functions and e Fig. 7 shows the invention adapted for usewith the target diagram apparatus and employing thermionic valves fordischarging the condensers.

Referring now to Fig. l the branch circuit consists of a rectifier MR inseries with a condenser C5 and means for discharging the condenser via aresistance Ri. such means "being, for instance. a contact or otherswitching device S2.

The nature of the voltage waveform .applied to the rectifier andcondenser will be, in general, of impulsive type; that is a rapid changefrom zero to a maximum with a more gradual decay thereafter to zero. Thepolarity of the rectifier will be such as to pass the voltage to thecondenser. In order to explain the action of the circuit element it willbe assumed that the input voltage/time function has been established ona condenser C, and that a contact or other switching device SI under thecontrol of the voltage/time function, switches the condenser on to aresistance R. The voltage waveform on R will be of the impulsive typeand is applied to the branch circuit. The contact or other switchingdevice S2 in the latter is assumed to be open at the moment Si closes,and therefore, i

a voltage is established on Cl via therectifier MR. This voltage willremain on CI for a time dependent on the time constant of Cl and theback resistance of the rectifier. This time constant can be made verylarge, so that for all 1 practical purposes the voltage on Cl can beconsidered as constant, and the circuit has performed the function ofstorage and delay by a time equal to the original voltage/time functionor pulse. When the next voltage/time function or pulse occurs SI is madeto open, and S2 is closed. The voltage on Cl then discharges impulsivelythrough RI, and if applied to a further similar branch circuit can bestored again and delayed by the time interval between the first andsecond pulses. By placing any number of such elements in series, aseries of pulses may be obtained in similar sequence to the original butdelayed by any time dependent on the time duration of, and intervalsbetween the original impulse series.

The contact or other switching device SI may not be necessary in generalas the voltage on R during the establishment of the voltage on C may beof such polarity as not to pass current through the rectifier. It isessential however, that the contact or other switching device in thebranch circuit (such as S2) shall open at or before the charging of theprevious storage condenser (such as Cl), and that the dissipation of thecharges on all condensers shall be substantially complete.

In order to transfer a reasonable fraction of the initial voltage on Cto Cl, it is necessary that Cl should not be greater than C and the timeconstant of Cl with the forward resistance of the rectifierMRshouldnotbe greater than that of C and R. An approximate idea of the voltagepassed on to C2 may be obtained by considering the forward resistance ofthe rectifier to be zero, when the charge on C is distributedinstantaneously between C and Ci, so that the voltage on Cl will beC/C-i-Cl of the original voltage on C. This value neglects theresistances of the condensers, which is in practice justifiable. It isalso found 'in practice that even when the time constant of Cl and theforward resistance of the rectifier is comparable with that of C andthat the forward resistance is zero can be obtained.

Figs. 2 and 3 show the application of the invention to the targetdiagram apparatus. In the form of this apparatus described in said priorpatent the delaying of the application of the voltages representing themake and break periods of the impulsing contact to the deflecting platesof the cathode ray tube was effected by stepping switches whose bankcontacts were connected to condensers on which were stored voltagesproportional to the logarithms of the make and break periods of theimpulses. By the present invention it is ossible to eliminate thestepping switches thereby removing a source of trouble as regards dirtycontacts, to reduce the number and size of the circuit components and topermit the use of low current supply voltages so that a compact mainsunit can be used.

The make period circuit shown in Fig. 2 will be first described as beingthe simpler of the two. A relay X, of the high speed type, is used torepeat the impulses which are to be tested, and has a changeover contactXI. The changeover or moving contact is connected to the positive end ofa battery, and the break contact (1. e. normally made) to the junctionof the charging resistance Ra: and the condenser C1, the other end ofRa: being connected to the negative end of the battery, and theremaining lead of C1: is connected via a discharge resistance R2 to thepositive end of the battery. Thus assuming that the break contactof XIis'broken for the time duration of the make period, C: will charge via aRs: plus R2 to a voltage which can'be made to represent the logarithm ofthe make period in the mannerdescribed in said prior patent. Thedischarge resistance is small compared to R1: so that when the breakcontact restores to normal the time constant of the discharge of C1:

via R2 is some 2 milli-seconds, compared with some 39 mllli-seconds forthe charging time constant of Ra: plug R2 and CI. The voltage waveformof the discharge will be of the impulsive type, and is applied to thebranch circuit. consisting of a rectifier MRI connected with itsnegative pole to the junction of C1: and R2 and its positive pole to acondenser C2 whose remaining lead is connected to the positive battery.To complete the branch circuit the junction of C2 and the positive poleof the rectifier is taken via a discharge resistance P to the makecontact of XI. Hence theimpulsive voltage on R! when C2: is discharged,is transferred as previously described via MRI to C! so that the latterremains charged to a voltage proportional to the initial voltage on C1:at the moment of discharge, for a time equivalent to the break period ofXI. The next make period will dissipate thls voltage via P and the makecontact in readiness for the next charge and discharge of Car. Thusthere will be available on C2 a I series of voltage pulses ofsubstantially constant B, some 60% of the value obtained by assuming 75amplitude proportional to the logarithm of the make periods of theimpulses, of time duration equal to the break periods of these pulses,and available at the end of the make periods.

The break period circuit is similar, the charge resistance being now Ry,the charge condenser Cu, discharge resistance R3, rectifier MR2 andstorage condenser C3, except for theconnections of a relay contact Y-lon a further high speed relay Y operated from the test impulses. Themake contact of Yl is connected to the junction of Cy and R11, so thatthe charge on C corresponds to the break period, whilst the breakcontact is connected via a discharge resistance R4 to the Junction of CIand MR2 positive.

Thus the voltage on C2 persists for the make period of YI and isproportional to the logarithm of the break period, and is availableafter the first break period. Since in most automatic telephone systemsthe impulsing sequence is a break followed by a make, the voltagerepresenting the break period must be delayed until the end of thefollowing make period before simultaneous use can-be made of the breakand make voltages. circuits used in the case of the break period will beone greater than the number used for the make period. The voltage on C2must therefore be delayed by another branch circuit so that it isavailable after the first make period. A further branch circuit isconnected to R4 therefore so that the impulsive discharge voltage of C2can be passed on to a further condenser C4 via another rectifier MR3 atthe moment when YI restores to normal, that is, after the first makeperiod. To complete the second branch circuit, the voltage on C4 isdissipated at the commencement of the next make period by a further makecontact BI and resistance Q, the changeover contact of BI beingconnected to the break contact of YI. The relay B, of similar type to Xand Y, is also operated from the test impulses.

It is now apparent that steady voltages are simultaneously available onC2 and C4 for the duration 'of the break period, and of respectivevalues proportional to the logarithm of the make and break periods ofthe impulses. They can therefore be applied to the deflection plates ofthe cathode ray tube over leads X and Y provided a common connection canbe found. The positive busbar gives such a connection since the breakcontact of YI will be made at the time these voltages are required.

It will be appreciated that in practice both the circuits of Figs. 2 and3 are energised from the same busbars A and B, which form two out ofthree busbars. The object of this three busbar scheme is firstly toobtain a bias to render the deflection zero when the standard impulsesare applied, secondly to obtain an adequate voltage to energise thecathode ray tube and finally to enable push-pull amplifiers to be used.

As mentioned in the general description of the branch circuit thenecessity for a contact or switching device preceding the first circuitelement, does not occur in this case, as the voltages on R2 and RIduring the establishment of the voltages on Ca: and C11 are of suchpolarity as not to pass current through the rectifiers MRI and MR2. thatthe charge dissipating contacts in the circuit elements (1. e. XI make,YI break, and BI make) shall open at or before the charging of thestorage condensers (i. e. C2, C2 and C4) will automatically be satisfiedin the case of XI and YI owing to the inevitable transit times of thecontacts, whilst in the case of BI this result can be secured byreduction of its release leg with respect toYI. The latter conditionwill. in general, be facilitated by theunnecessary ad- Justments to theY relay in order to make the YI make contact distortionless, since its.release leg will have to be increased to compensate for the transittime.

Hence the number of branch In addition the first requirement dissipationof the charges on the condensers shall be substantially complete, iscatered for by making the discharge time constants (i. e. Ca: and R2, C2and P, Cu and R2, C2 and R4 and C4 and Q) not greater than 2milli-seconds. As the minimum discharge period will be the make timewhich may be as low as 10 milli-seconds, this time constant will reducethe original charge to 0.005 of its value in 10 mill-seconds, which isquite satisfactory.

The frequency of operation of the circuit elements may be increased byreplacing the contacts by switching devices such as thermionic orgas-filled valves, controlled by the, incidence and polarity ofimpulsive voltage inputs. Fig. 4 shows a type of circuit arrangementadapted for use with-a voltage/time function of impulsive nature inwhich a positive half-wave is followed by a complementary negativehalf-wave. The circuit employs four storage and delay circuits I, II,III and IV in which four thermionic valves VI, V2, V3 and V4 are used,the valves being alternatively switched by the incidence of nega tiveand positive wave-fronts applied to the input terminals I and 2. Thevoltages on the impedance Z, which are to be transferred along the chainof branch circuits, are made to represent the desired voltage/timefunction in any wellknown manner. The circuit operates in the followingmanner. The voltage/time function is applied to the terminals I and ,2and when ter-- minal I is positive with respect to. terminal 2. thecondenser Cl is charged. The negative potential on terminal 2, however,makes the grids of the valves VI and V2 negative with respect to thecathode so that these valves are closed i. e. in a non-conductingcondition to prevent rectifiers MR4 and MRI in conjunction withcondensers CI and 01 from discharging. When terminal 2 is made positivewith respect to terminal I, valves VI and V! are opened i. e, madeconducting so that condenser Cl discharges through the valve VI andresistance RI2 to cause the charging of the condenser C4. The charge ismaintained on the condenser C8 owing to the rectifier MRI and owing tothe fact that the valve V2 and also the valve V4 are closed due to thenegative potential on the grids of the valve. when the next voltage/timefunction arrives at terminals I and 2, terminal I is again made positivewith respect to terminal 2 so that condenser C4 discharges andcondensers Cl and C1 are charged. In this manner successive voltage/timfunctions are transferred successively from one circuit to the next. Itwill be appreciated that the number of circuits may be added to, whilstthe output may be taken between terminals 8 and 4, or 4 and I,

- according as to whether an impulsive or substan- The secondrequirement that the -Il m tially steady output is required.

As previously explained it is an essential requirement of the branchcircuits that its switching device shall open at or before the chargingof the previous storage condenser, that is, preferably before theclosure of the previous switching device. This can readily be achievedin the case of thermionic switches by delaying the application of thepositive grid potentials which close the valve, with respect to thenegative potentials which open the valve. A typical circuit is shown inFig. 5, in which voltage wavefronts applied to M and N, and making I!positive to N are only applied to the grid/cathode path of the valveafter a time dependent on the time constant r2 and cIl' (the rectifier Jbeing when the voltage wavefront makes M negative to N, the greaterportion of it is immediately applied to the grid via Ml, across therectifier and resistance H, the latter being needed so that cill maydischarge during the negative pulse in readiness for the next positivepulse.

It will be noticed that in all the foregoing description of the actionof the branch circuit, the voltage pulses available at the end of achain of circuits'only last for either the duration of the originalvoltage/time functions or the time interval between such voltage/timefunctions, according to the number of circuits employed. In certaincases where the indicating instrument has a time lag, such as forinstance an ammeter, the desired readings may be confused by the motionof the instrument tending to, or returning from zero- In such cases thepulse time, or time interval between pulses, is comparabl with the timelag of the meter, and the terminating circuit element may be switched asshown in Fig. 6. The voltage/time function is applied to Al and BI andthe desired representation of it in the form of an impulsive voltage isgenerated across the impedance Zl in th normal manner, and transferredvia any number of branch circuits between E and F to a low impedancesource L which may be momentarily shorted without affecting the inputvoltage. A typical form of such source is a cathode follower valve witha high resistance in the grid lead; in this case L represents thecathode resistance. The pulse voltages on L are passed on to C9 viarectifier MR8, and are available at terminals '1 and U for operation ofthe meter. The thermionic valve V5 is arranged to short C9 only for thetime necessary to dissipate substantially its voltage via the internalresistance of VI. This time depends on the time constant of C9 and V5and may be made a fraction of a miili-second, if desired, In order thatthis time may not, be exceeded, the positive grid pulse isdifferentiated by the grid resistance Gi and condenser Kl, so that avery short positive grid pulse is obtained; it is obvious that the timeconstant of Ki and GI must be of the same order as C9 and V5, or higher,depending on the form of the applied voltage/time waveform. Both thecondenser C9 and impedance L will be shorted momentarily, therefore, andhence the need for a source which can be shorted without affecting theinput voltage.

When the short is removed, C9 will charge up again to the new inputpulse voltage, and hence the cut-put across '1 and U will consist of aseries 01" voltages of nearly the same amplitude as would have formerlybeen obtained, but separated only by a very short time gap. The meterwill not, of course, respond to these gaps and the indication willtherefore be much steadier. A source of grid bias, such as a battery 132will in :e erel L:- needed. so as to prevent the valve from n'ghestvoltage desired on on to the grid condenser 11;," point in the precedingWavefront exists with at the moment a to L. It be end of C3, in oer n3effect over some 200 milli-seconds being effected by an electricalcircuit, instead of by the meter movement. A quicker and less tiringestimate of the input level is obtained.

It is obviously practicable to use thermionic switching, as describedabove, for the target diagram apparatus, with the advantage that therelays can be eliminated together with the necessity for specialadjustments to secure distortioniess operation. Fig. 7 gives a typicalcircuit arrangement, in which the main components used for measuring thebreak and make periods and the storage and delay groups are lettered tocorrespond with Figs. 2 and 3.

The impulsing contact TI to be tested, is con nected via a positivebattery B3 to the input resistance R33 of a valve V6, so that closure ofTI increases the anode current, grid current being limited by R32. Themake period charge condenser Cr is thereby caused to discharge'from theprevious anode voltage towards the much lower voltage on V8, due to theincrease of anode current, via resistances R2, Rx, R23 and V (therectifier MRI! being non-conducting). Of these resistances it isarranged that R1: shall have predominating effect. At the termination ofthe make period, V6 becomes a very high resistance, so that C: chargesfrom the busbar voltage and via R22, R2 and the forward resistance ofMRI2, within say 2 miili-seconds. The resultant voltage pulse on R2 ispositive to the grid of V1 and of magnitude corresponding to thelogarithm of the make period in the usual manner. V1 is designed so thatit acts substantially as a cathode follower, so that the voltage on R25is in phase and of approximately the same magnitude as the input gridpulse (grid current produced by the fortuitous long make periods beinglimited by RSI). The voltage pulse on R25 is applied to the usualcircuit MRI, C2, and discharge valve Vl, so that the make periodvoltages are available between terminals 6 and l. The switching of V! atthe beginning of the next period is obtained by applying the impulsivevoltage change on the anode resistance R24 of V1, due to a negativepulse applied to the grid across R1: and R2 during the discharge of CI,to the grid of V8 via a condenser CI2 and resistance RIB. Provided thelatter time constant slightly exceeds that of C2 and V8, and the latteris not greater than some 2 milli-seconds, C2 may be thus substantiallydischarged in the minimum pulse time of 10 milliseconds. A grid bias BIis needed for V8 sumcient to prevent conduction when C2 is charged tothe maximum test voltage.

The circuit for the break period is of similar nature, except that twobranch circuits are used, and the discharge valve Vii must include aresistance RlB, in order to pass the voltage on to the second element.The discharge of C4 is eiiected by applying a positive pulse to the gridof VI2 from R29, since at this moment Vii is nonconducting. The inputcharge and discharge valve V9, corresponding to V6, is operated from thelatter by a. high resistance potentiometer R24, R25, since Cy isrequired to discharge via Ry at the end of the make period, 1. e. at;the beginning of the break period.

It will be noticed that the difficulty previously mentioned when usingthermionic discharge valves, that valve shall open at or befoie thecharging of the plBViOLlS storage condenser, has been complete,-overcome without using the cir cuit shown in Fig. 5, making thedischarge eiifect impulsive, so that the previous gondenser isdischarged and open clrcuited before the next pulse. Thismethod is ofcourse superior to the former method, since it can invariably be usedwhatever the nature of the available voltages for causing the dischargevalves to function.

In certain circumstances, the condensers C2 and C! may be placeddirectly in the cathode leads of V1 and VII, thus eliminating RII, R",MRI and MRI.- I'he grid/cathode path takes the place of the rectiflers.This method has the advantage of giving a greater voltage output on 1 C2and C3, by causing the condensers to charge from the supply voltageduring time grid current iiows. MRI can likewise be replaced by a valvewith C4 in its cathode lead. 1

It will be appreciated that since the circuit in Fig. 7 measures thehalt-cyclic periods of the,

input waveform, it may be used as a frequency meter, by designing it foroperation at the highest required irequency, and causing VI to act as a"peak chopper." By using a cathode ray tube as the indicator, the"instantaneous Irequencyoi' theinputmayberead.sothat, for instance, therrequency spectrum oi speech or music may be displayed; The remainingco-ordinateaxisoi'thetubemaybeused forthe peak value or time duration ofthe input.

I claim:

1. A circuit arrangement comprising a main circuit in which directcurrent impulses are generated, a series or branch circuits tedtherewith, certain of said branch circuits having means responsive tothe make period 0! mid impulses to produce voltages proportionalthereto, and others of said branch circuits having means rsponsive tothe break period'oi' saidimpulses to produce voltages proportionalthereto, each branch circuit including a rectifier and a cond nser inseries and means for the condenser after a certain tune interval; saidmeansforthecondensersintheflrst group 01' branch circuits beingeflective substantiaily at the termination of a subsequent make periodto pass on the voltage to the next branch circuit, the means fordischarging the condensers in the second group of branch circuits iseii'ective at the termination 01' a subsequent break period to pass thevoltage to the next branch circuit.-

2. A circuit arrangement as claimed in claim 1, in which the means fordischarging said condensers comprise thermionic tubes each connectedacross one of the condensers, the voltage of the grids of the valvesbeing controlled by-the impulses to render the tubes conducting or nonconducting to control-the discharge or the condensers.

-3'. A circuit arrangement such as claimed in claim 1 in 'whichthere arethermionicvalves for controlling the discharge oi. the condensers. meansfor placing positive voltage on the grid of the tubes and for thenplacing negative voltage thereon, and mean for delaying the applicationor positive voltage to a tube to ensure that that tube is madeconductive at or beiore the previous tube is made non conductive.

4. A circuit (arrangement for simultaneously obtaining voltagesproportional to the make and break periods or an impulsing contactincluding a main circuit having the contact therein and two groups ofbranch circuits associated therewith, a pair of thermionic valvesassociated with the main circuit, a condenser and a uni-directionaldevice in each branch circuit, means for causing the first one of saidvalvesto pass current to cause charging of the condenser in the firstbranch circuit oi! one group when thecontacts make, said second valvehaving an input comprising a potentiometer connected across the firstvalve, and means tor causing the second valvetopasscurrent-tocausecharsingofthecondenserintheflrstbranchcimiitortheother the break of said contact.BERTRAM MORTON HADFIELD.

